The pronunciation of the letter v is one of the few cases of ambiguity in German orthography. The German language normally uses the letter "f" to indicate the sound /f/ (as used in the English word fight) and "w" to indicate the sound /v/ (as in victory). However, the letter "v" does occur in a large number of German words, where its pronunciation is /f/ in some, but /v/ in some other words.
In mediaeval German, the sound /f/ had been voiced in some dialects and was therefore written "v". Contemporary German has gone back to the unvoiced pronunciation, but kept the mediaeval spelling here and there. As a general (and defective) rule, we can thus say that "v" is pronounced /f/ in originally German words, and /v/ in words of foreign origin.
Rules
V is pronounced /f/
V is pronounced /v/ (or /ʋ/)
Pronunciation varies between /f/ and /v/